triangle.frag

// Author:
// Title:

#ifdef GL_ES
precision mediump float;
#endif

uniform vec2 u_resolution;

bool insideTriangle(vec2 uv) {
    vec2 a = normalize(vec2(0.0, 0.0) - uv);
    vec2 b = normalize(vec2(0.5, 1.0) - uv);
    vec2 c = normalize(vec2(1.0, 0.0) - uv);
    
    return dot(a, b) + dot(b, c) + dot(a, c) < -1.0;
}

void main() {
    vec2 uv = gl_FragCoord.xy/u_resolution.xy;
    
    float red = uv.y;
    float green = (1.0 - uv.x) * (1.0 - uv.y);
    float blue = uv.x * (1.0 - uv.y);
    
    gl_FragColor = insideTriangle(uv) ? vec4(red, green, blue, 1.0) : vec4(1.0);
}